This invention relates to coal liquefaction and is particularly concerned with catalytic hydrogen-donor coal liquefaction.
Processes for the direct liquefaction of coal and similar carbonaceous solids normally require contacting of the solid feed material with a hydrocarbon solvent and molecular hydrogen at elevated temperature and pressure to break down the complex high molecular weight starting material into lower molecular weight hydrocarbon liquids and gases. The most promising processes of this type are those carried out with a hydrogen-donor solvent which gives up hydrogen atoms in reaction with organic radicals liberated from coal or other feed material during the liquefaction step. In such a process, the hydrogen-donor solvent is subsequently regenerated in a downstream solvent hydrogenation step. Plants for carrying out processes of this type normally include facilities for generation of the needed molecular hydrogen by the gasification of heavy liquefaction bottoms produced in the liquefaction step, by the coking of liquefaction bottoms and subsequent gasification of the resultant coke, by the reforming of light hydrocarbon liquids and gases produced in the process, or by other means.
It has been suggested in the past that liquefaction processes can be improved by the use of hydrogenation catalysts in the liquefaction or reaction zone. Conventional hydrogenation catalysts that have been used for such purposes include cobalt-molybdenum, nickel-molybdenum and nickel-tungsten supported on alumina, silica-alumina and similar materials. Such hydrogenation catalysts have been used in both nondonor and hydrogen-donor solvent systems.
Although conventional hydrogenation catalysts of the type referred to above are reasonably effective in increasing yields from liquefaction processes, experience has shown that such materials are not well suited for use under liquefaction conditions because their activity is drastically decreased by the deposition of carbon and mineral matter on the surface and in the pores of the catalyst particles. Because of the deactivation caused by the severe temperature and pressure conditions extant in the liquefaction reactor, conventional hydrogenation catalysts are only effective for short periods of time and must be frequently replaced in order to maintain hydrogenation activity.